Opiates have been the subject of intense research since the isolation of morphine in 1805, and thousands of compounds having opiate or opiate-like activity have been identified. Many opioid receptor-interactive compounds, including those used for producing analgesia (e.g., morphine) and those used for treating drug addiction (e.g., methadone, buprenorphine and naltrexone) in humans work by triggering μ opioid receptors in the central nervous system (CNS) and by crossing the blood-brain barrier. However, as there are μ opioid receptors outside the CNS, these opiates usually cause unwanted peripheral side effects. Often, the peripheral side effects manifest themselves in the gastrointestinal (GI) tract and the respiratory system. For instance, prolonged morphine administration often causes constipation, and prolonged morphine administration ultimately causes life-threatening respiratory depression in patients. Other side effects appear to arise from the central action of morphine-like compounds. These central side effects of μ ligands include physical dependence (addiction) and sedation. Thus, a drug that is able to treat symptoms of pain, but not cause some or all of the peripheral and central side effects, would be most valuable.